pwd     # print working directory

ls     # list content

ls /dir/subdir     # starts with / -> absolute path

ls dir/subdir     # doesn't start with / -> relative path

ls -R    # see also content in the subdirectory

ls -F     # see also a * after runnable programs, / after directories

cd     # change directory

..     # parent directory

cd ..     # go to parent directory

ls ..     # list parent directory content

~     # home directory

cd ~     # go to home directory

ls ~     # list home directory content

cp     # copy files

cp original.txt duplicate.txt     # copy original.txt and name it duplicate.txt

# NB: use quotes if there are spaces in files or directories names

cp original.txt duplicate.txt backup     
# copy the 2 files in the backup directory

mv     # move or rename files or directories

mv backup/original.txt backup/duplicate.txt ..     
# move the 2 files from backup to the parent directory

mv original.txt old.txt     # rename original.txt in old.txt

mv backup backuptwo     # rename the backup directory

rm     # remove files

rm old.txt duplicate.txt     # remove the 2 files

rmdir     # remove a directory only if it's empty

rmdir backuptwo

mkdir     # make a new directory

mkdir backup

cat     # print file content

cat test.txt

less     # display file content one page at time

less test.txt
# now we can use spaceboard for go to next page or q to quit

less test.txt new.txt

# :n for go to the next file, :p for go to the previous one, :q to quit

head     # display first 10 lines

head -n 3 test.txt     # only first 3 lines

# NB: with tab you have auto completion, double tab in case of ambiguity

man     # find out what a command do

man less

cut      # select column from a file

cut -f 2-5,8 -d , file.csv     # -f = fields, -d = delimiter
# select columns from 2 to 5 and 8 using comma as separator

grep     # select a line in file according to what contain

grep hello file.csv     # select lines with hello

# grep patterns
# -c: print a count of matching lines rather than the lines themselves
# -h: do not print the names of files when searching multiple files
# -i: ignore case (e.g., treat "Regression" and "regression" as matches)
# -l: print the names of files that contain matches, not the matches
# -n: print line numbers for matching lines
# -v: invert the match, i.e., only show lines that don't match

grep -n -v hello test.csv     # select lines with lines number without hello

grep -c hello file.csv two.csv     # how many lines with hello in the two files 

sed     # replace

sed 's/hello/hi/g' test.txt     # replace hello with hi in test.txt

>     # redirect output to a file

head -n 3 test.txt > test2.txt

|     # create pipe

head -n 9 test.txt > tail -n2
# take first 9 rows, then take the last two of the result (8-9)

cut -d , -f 1 dir/file.csv | grep -v Date | head -n 10
# 1. select first column of the comma delimited file.csv in dir
# 2. remove lines with "Date" (maybe the header)
# 3. take the first ten lines

wc     # count of the character (-c), words (-w), lines (-l) in a file

grep hello file.csv | wc -l     # num of records with hello

# wildcards
# * -> matches 0 or more characters
# ? -> matches a single character
# [...] -> matches any one of the characters in the brackets
# {...} -> matches any of the comma-separated patterns in the brackets

cut -d , -f 1 dir/*.csv     # first field of all csv files in dir

cut -d , -f 1 dir/c*.csv     # first field of all csv files in dir starting with c

sort     # put data in order (alphabetically without patterns)

# sort patterns

# -r -> reverse alphabetical order
# -r -> sort numerically
# -b -> ignore leading blanks
# -f -> case insensitive

cut -d , -f 2 dir/file.csv | grep -v hello| sort -r
# take column 2 of the file, only the lines with hello, reverse order

uniq     # remove adjacent duplicate lines

cut -d , -f2 file.csv | sort -r | uniq
# take second column of the file without duplicates

cut -d , -f2 file.csv | sort -r | uniq -c
# same but with count of how often each occours

cut -d , -f2 file.csv | sort -r | uniq -c > new.csv
# same the result in new.csv

# ctrl+c for stop a running program

# some environment variables
# HOME -> User's home directory
# PWD -> Present working directory
# SHELL -> Which shell program is being used
# USER -> User's ID

echo     # prints its argument

echo hello     # print hello

echo $USER      # print the value of the variable

testvar=dir/file.csv    # assign dir/file.csv to the variable testvar

head -n 1 testvar

# expr for numeric calculation but without decimal

expr 1+3

# bc is a calculator program, you can use it in a pipe

echo "5 + 7.5" | bc

echo "scale = 3; 10 / 3" | bc     # scale for how many decimals

# for loops
for [variable] in [list] ; do [body] ; done

for file in dir/*.csv; do head -n 2 $file | tail -n 1; done

# print second line of each csv in directory dir

nano file.txt     # edit file.txt with nano text editor

history     # see your command history

history | tail -n 3 > steps.sh    # save your last 3 steps to file

head -n 1 dir/*.csv > header.sh     # save command in sh file

bash headers.sh     # tell shell to run commands in the file

$@     # all of the command-line parameters given to the script

# Es. if unique-lines.sh contains sort $@ | uniq, executing:
bash unique-lines.sh dir/file.csv
# will run this command:
sort dir/file.csv | uniq

# if you execute this

bash unique-lines.sh dir/file.csv dir/file2.csv 

# it processes both the files

# use $1, $2, and so on to refer to specific command-line parameters
head -n $2 $1 | tail -n 1 | cut -d , -f $3 > get-field.sh
# take a filename, the row to select, the column to select, and print
bash get-field.sh dir/file.csv 4 2

# you can write for loops in shell scripts without semicolon
# Print the first and last data records of each file.
for filename in $@
do
    head -n 2 $filename | tail -n 1
    tail -n 1 $filename
done

# in shell scripts use # for comments

# use \ to go to new line